13. NANKAI TROUGH AND JAPAN TRENCH LATE CENOZOIC PALEOENVIRONMENTS
DEDUCED FROM CLAY MINERALOGIC DATA1
Hervé Chamley, Sédimentologie et Géochimie, Université de Lille I
and
Jean-Paul Cadet and Jacques Charvet, Sciences de la Terre, Université d'Orléans2
ABSTRACT
The clay mineralogy and morphology of DSDP Leg 87 sediments, studied from 200 samples and considered together with data from DSDP Legs 57 and 58, are discussed and interpreted in terms of paleoenvironment. Terrigenous influence represents the main factor responsible for clay deposition in the Nankai Trough and Japan Trench, both marked by
proximity to Japan, high submarine relief, and high sedimentation rates. The argillaceous diagenesis is of local or minor
importance and includes the readjustment of fleecy to lathed smectites. The volcanic influence consists chiefly of a reworking of smectite developed by subaerial alteration of island arc rocks and secondarily of a local alteration of ash
beds.
The late Cenozoic clay sedimentation in the Nankai Trough (Sites 582 and 583) underwent a major change during the
early Pleistocene. Hemipelagic sediments characterized by a wide range of minerals, derived from various sources characterizing the whole northern Philippine Sea (Leg 58 data), were replaced by turbiditic sediments with clay associations
typical of a restricted southern Japanese origin (chiefly illite and chlorite). The mineralogic transition, which roughly
parallels the lithologic one, occurs by short successive steps. It appears to express indirectly the tectonic activity of the
southern margin of Japan, marked by the subduction of the Shikoku Basin and the formation of the Nankai Trough.
Middle Miocene to Pleistocene sedimentation on the inner part of the Japan Trench (Site 584) corresponds to a regular detrital supply from Japan, with dominant smectite associated with various minerals. Small mineralogic variations
occur, determined by the late Cenozoic climatic variations and possibly by the volcanic activity. The Site 584 clay stratigraphy parallels the successions recorded at Sites 438 and 440 (Leg 57), without significant evidence of differential settling. The first deposits recovered at Site 584 correspond to the end of an important geodynamic history, well recorded
in the clay stratigraphy of Sites 438 and 439: (1) existence during the Late Cretaceous of a sloped continental landmass,
located somewhere east of the drilling area; (2) proximity of the actively eroded "Oyashio landmass" during the late
Oligocene; (3) probable initiation of the collapse of this landmass during the latest Oligocene after a strong stage of volcanic activity; and (4) increase of the influence of Japan on sedimentation during the early-middle Miocene.
INTRODUCTION
The Leg 87 drilling program was devoted to the exploration of two Japanese trench complexes (Fig. 1; Leg
87 Scientific Party, 1983; site chapters, Sites 582-584,
this volume). The Nankai Trough was drilled at two places, about 8 km apart. Site 582 (31°46.5'N, 133°54'E;
three holes; water depth, 4892 m; maximum penetration, 749.4 m) is located in the undeformed sediments
of the trench floor, about 2 km south of the deformation front revealed by seismic profiles. About 560 m of
Quaternary trench-fill (turbiditic) sediments overlie lower Quaternary and upper Pliocene hemipelagites of the
Shikoku Basin. Site 583 (31°5O'N, 133°51.3'E; eight
holes; water depth, 4663-4673 m; maximum penetration,
450 m) is located through the lowest structural terrace of
the inner slope of the Nankai Trough: Holes 583B,
583C, and 583D at the toe of the basal thrust; Holes
583E, 583F, and 583G 500 m landward; and Hole 583A
still 400 m more landward, crossing the uppermost part
40°N -
30°
1
Kagami, H., Karig, D. E., Coulbourn, W. T., et al., Init. Repts. DSDP, 87: Washington (US. Govt. Printing Office).
2
Addresses: (Chamley) Sédimentologie et Géochimie, ERA 764, Université de Lille I,
59655 Villeneuve d'Ascq, France; (Cadet, Charvet) Sciences de la Terre, ERA 601, Université
d'Orléans, 45046 Orleans, France.
130°E
135°
140°
Figure 1. Location of DSDP Sites: Legs 57 (Sites 438 to 440), 58 (Sites
442 to 444), and 87 (Sites 582 to 584).
633
H. CHAMLEY, J.-P. CADET, J. CHARVET
of the section missing in the other holes. Sediments consist of Quaternary dark gray hemipelagic muds with frequently graded sand and silt layers, and with sparse ash
and vitric sand. Site 584 (40°28.0'N, 143°57'E; three
holes; water depth, 4124-4152 m; maximum penetration, 954 m) is situated on the deep-sea terrace of the
trench slope, approximately 42.5 km upslope from the
Japan Trench axis. Sediments consist of thin Pleistocene
and lower Pliocene diatomaceous mud and mudstone
(0-231 m); lower Pliocene to upper Miocene diatomaceous mudstone with fine sand and silt beds dipping
seaward (231-537 m); and upper to middle Miocene
varicolored and bioturbated mudstone with a much reduced diatom content.
METHODS
The clay mineralogy of 204 samples, regularly located along the
Leg 87 cores (69 samples from Site 582; 83 from Site 583; 52 from Site
584), has been studied by X-ray diffraction on < 2 µm, oriented pastes.
Some selected samples of the different mineralogic units identified
have been observed by transmission electron microscopy. Methods have
been recorded by Chamley (1980a). The purpose of the study was to
participate in reconstructing past environments from clay mineral stratigraphy. Therefore, the results from Leg 87 will be presented first and
then discussed together with previous data obtained from Legs 57 (Japan Trench; Chamley and Cadet, 1981) and 58 (Shikoku Basin; Chamley, 1980b; White et al., 1980).
CLAY MINERALOGY OF SITES 582, 583, AND 584
Nankai Trough
Most of the Pleistocene sediments studied at Sites 582
(Cores 582-1, 582A-1, 582A-2, 582B-2 to 582B-54) and
583 (all cores) contain very similar clay assemblages
throughout the stratigraphic column, and the data are
easily expressed as average values (Fig. 2; Plate 1, Figs.
1, 2). Chlorite (20-35% of the clay fraction) and illite
(30-50%) are abundant; smectite is fairly abundant (1035%); irregular mixed-layers (chiefly of illite-smectite
and chlorite-smectite types) and kaolinite are minor components (5-10%). With regard to associated minerals,
quartz and feldspars are common, and amphiboles are
rare to fairly common. An example of this homogeneity
arises from Holes 583B, 583C, and 583D, drilled one
below the other at the same location down to 326.6 m
(Fig. 3). The only significant change recorded concerns
Cores 583D-14 to 583D-16, where the smectite abundance reaches 35% of the clay fraction. Holes 583, 583A,
and 583E to 583G are marked by assemblages quite similar to these average ones.
The lower Pleistocene and Pliocene sediments of Cores
582B-55 to 582B-73 contain higher amounts of smectite
(25-40% of the clay fraction) than in overlying deposits
Site
Cores
1
A1, A2
582 B2 to B54
Pleistocene
(49 samples)
B55 to B73
Pleistocene-Pliocene
(20 samples)
(10-25) (20-35) (8-17)
1 to 26 - A1 to A11
B1 to B5 - C2 to C5
D3 to D29 - F6 to F29
583 G1 toG15
Pleistocene
(83 samples)
584
1 to 93
e. Pliocene,
I. to m. Miocene
(48 samples)
(10-30)(15-25)(10-15)
(40-55)
94 to 97
middle Miocene
(4 samples)
/ (10-15) \
(5-10) (5-10)
Chlorite ES3 Illite
(65-75)
Smectite
1 Kaolinite
o Rare
• Fairly common
O Common
• Very common
Figure 2. Average clay composition of Leg 87 main mineralogic units. In the Cores column, letters before
numbers indicate the hole (e.g., Al, A2 = Cores 582A-1 and 582A-2). If there is no letter, that indicates the first hole of that site (e. g., 1 to 26 = Cores 583-1 to 583-26). Numbers in parentheses in the
Clay Mineralogy column show the extreme percentages.
634
LATE CENOZOIC PALEOENVIRONMENTS BY CLAY MINERALOGY
Nankai Trough
Core-Section
(level in cm)
1-2,0
2-3, 69
3-2, 3 0 ^
4-3, 58"^
5-1,69
2-4, 2 8 /
3-3, 3 r
5-1,111
3-1,38
4-1,40
5,CC
6-2, 60
7-1,45
8-1,80
9-2, 30
10-1,38
11 - 1 - 6 1
1 2-2, 64
13-1, 80
14-3,40
15-2,40
16-1,60
17-1,50
19-2, 48
20-1, 15
gg|g|| 22-3, 1 28
24-2, 90
25-1,64
300 H
S3S3S
29-2, 55
Figure 3. Detailed clay mineralogy of Pleistocene sediments of Holes 583B, 583C, and
583D, Nankai Trough. For key, see Figure 2. For lithologic symbols, see Explanatory
Notes chapter (this volume).
(Figs. 2 and 4). The expandable mineral typically consists of fleecy or lathed particles (Plate 1, Figs. 3, 4) and
corresponds to a decrease of quartz and feldspar abundance and to the absence of amphiboles. Irregular
mixed-layers are fairly abundant (8-17%) and diversified (including vermiculitic and smectitic types), whereas the abundances of chlorite (10-25%) and illite (2035%) are lower than in more recent Pleistocene deposits.
Japan Trench
The lower Pliocene to upper middle Miocene sediments of Site 584 (Cores 584-1 to 584-93) contain homogeneous assemblages, characterized by abundant fairly
well crystallized smectite (40-55% of the clay fraction),
associated with fairly abundant chlorite (10-30%) and
illite (15-25%), some mixed-layers (10-15%, mostly smectitic types), and kaolinite (traces to 5%). Associated minerals include common quartz and feldspars, and common to abundant opal, essentially of biogenic origin
(Figs. 2 and 5; Plate 1, Fig. 5).
In the lowermost part of Hole 584 (middle Miocene,
Cores 584-94 to 584-97), smectite abundance increases
markedly (65-75% of the clay fraction), and other minerals are present in small amounts (especially kaolinite,
quartz, and feldspars). Smectite particles present both
fleecy and lathed structures. Biogenic silica, frequently
as abundant as in the overlying units (Plate 1, Fig. 6), is
probably responsible for the poor crystallinity of the minerals observed by X-ray diffraction on oriented pastes.
ORIGIN OF CLAY MINERALS IN NANKAI
TROUGH PLEISTOCENE SEDIMENTS
The illite-rich and chlorite-rich assemblages, which
characterize most of Pleistocene sediments at Sites 582
and 583 (Figs. 2 and 3), typically proceed from the erosion of igneous, metamorphic, and pre-Cenozoic sedimentary rocks, widely outcropping on the southern Japanese islands and responsible for a large part of the sedimentation in peri-Japanese seas (Aoki and Oinuma, 1974;
Aoki et al., 1974; Huang and Chen, 1975; Murdmaa et
al., 1977; Kolla et al., 1980; Chamley, 1980b). The remarkable abundance in Nankai Trough of illite and chlorite, associated with noticeable amounts of irregular
mixed-layers, quartz, feldspars, and amphiboles, points
to the large influence of southern Japan, where these
minerals are abundantly present in rocks and superficial
soil formations (Sudo and Shimoda, 1978). Such a strong
correspondence between the petrography of the subaerial rocks and the clay mineralogy of the adjacent marine sediments is in agreement with the lithologic and
geomorphologic data, showing that the Quaternary sediments at Sites 582 and 583 are chiefly turbidites, essentially fed axially from a source near the Japanese Izu pe-
635
H. CHAMLEY, J.-P. CADET, J. CHARVET
Rare
Common
Abundant
Figure 4. Clay mineralogy at the hemipelagite-turbidite boundary of Hole 582B lower Pleistocene sediments. For key, see Figure 2.
ninsula, about 400 km northeast of the drilling area
(Leg 87 Scientific Party, 1983). The smectite and kaolinite associated with the illite and chlorite group are also
chiefly supplied by turbiditic currents from Japan,
where they do exist in various rocks and soils (Kobayashi et al., 1964; Aoki and Oinuma, 1974; Aoki et al.,
1974; Sudo and Shimoda, 1978). The minor changes observed in the mineral percentages along the stratigraphic
column probably result from variations in the turbiditic
regime and from the Quaternary alternations of climate
determining the continental pedogenic alterations
(Chamley, 1980b; 1981).
INFLUENCE OF SEDIMENTARY REGIME, DIAGENESIS, VOLCANISM, AND TECTONICS ON
LATE CENOZOIC CLAY SEDIMENTATION
IN THE NANKAI TROUGH
The lower Pleistocene to Pliocene sediments at the
base of Site 582 (Cores 582-55 to 582-73) contain more
diversified clay assemblages than the overlying Pleistocene deposits (Figs. 2 and 4). The oldest clay associations at Site 582 strongly resemble those of late Cenozoic sediments at Sites 442, 443, and 444, located south of
the Nankai Trough in the Shikoku Basin (Chamley,
1980b). The common characteristic of these sediments is
their hemipelagic depositional regime, corresponding to
the supply in the northern Philippine Sea of minerals
from different major sources (Fig. 6): illite and chlorite
from Japan and eastern Asia; smectite from volcanic
arcs; irregular mixed-layers from various weathered soil
profiles; kaolinite from Southeast Asia and also Japan;
and vermiculite and sometimes palygorskite (attapulgite) from Asia. As a consequence, the region of Nankai
636
Trough appears to have been dependent, until the early
Quaternary, on the general sedimentary regime prevailing in the northern Philippine Sea, a regime marked by
open circulation of water masses leading to the mixing
of fine detrital minerals from various sources. The major change to a turbiditic sedimentary regime, occurring
at about 0.65 Ma (Leg 87 Scientific Party, 1983), is clearly expressed by the clay mineral assemblage and corresponds to a strong geographic restriction of the detrital
input, the Japanese islands remaining nearly the only
source of clay minerals. Clay associations appear to represent useful markers of the major modifications occurring in the terrigenous sources, responsible for a large
part of the late Cenozoic marine sedimentation in the
Nankai Trough area.
The major change in the early Pleistocene sedimentation, that is, a shift from a hemipelagic to a turbiditic
regime, probably resulted from tectonic activity in the
northern Philippine Sea. In particular, this activity was
marked by northward subduction, thrusting, and the formation of the Nankai Trough. Sedimentation expresses
this tectonic activity in different ways: unhealed fractures dipping near 40°, dip-slip grooves and striae, dewatering veinlets (Karig et al., 1983; Leg 87 Scientific
Party, 1983; site chapters, Sites 582 and 583, this volume). As far as the clay sedimentation is concerned, the
only obvious expression of this tectonic activity appears
to be the change from various terrigenous sources to a
single source, occurring during the early Pleistocene
(Site 582, close to 560 m) and corresponding to the
modification in the depositional regime and probably to
the deepening of the Nankai Trough. Note that this
change was not instantaneous, but marked by several
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H. CHAMLEY, J.-P. CADET, J. CHARVET
NATURE OF LATE CENOZOIC MINERAL
SEDIMENTATION IN THE JAPAN TRENCH
45°N
C: Chlorite
I: Illite
ml: Mixed-layers
V: Vermiculite
Sm: Smectite
K: Kaolinite
P: Palygorskite
30°
Site 443
Site 444
Site 442
Philippine Sea
15C
135°W
150°
165°
180°
Figure 6. Main sources of clay minerals in the Shikoku Basin (from
Chamley, 1980b).
steps, with the smectite amount decreasing and the illite
group increasing irregularly upwards from Core 582-59
to Core 582-54 (Fig. 4).
A general lack of other evidence of tectonic activity
revealed in the clay sedimentation is in agreement with
the low subduction rate and the subtle and limited smallscale structures in the sedimentary column itself (Karig
et al., 1983). The thrust surface crossing Hole 583D near
160 m, and deduced from the seismic reflection profile
(Leg 87 Scientific Party, 1983), could correspond to the
only significant mineralogic change in the sedimentary
successions in Holes 583B, 583C, and 583D: Cores 14,
15, and 16 of Hole 583D show a moderate increase of
smectite abundance, between about 170 and 200 m below mudline (Fig. 3). This temporary variation could
result from an increase of the volcanic activity or a lesser
dependence on supply from Japanese sources and could
also be contemporary to an epeirogenic destabilization.
The existence of a hanging-wall splay intersecting Hole
583D near 60 m sub-bottom and of a structural terrace
at Hole 583A only (also deduced from the reference
seismic profile) is not corroborated by any peculiarity of
the clay sedimentation.
The relatively high amount of smectite in the Pliocene to lower Pleistocene sediments of Site 582 probably
depends on the active volcanism characterizing the island arcs surrounding the Shikoku Basin during the late
Cenozoic (Huang and Chen, 1975; Murdmaa et al., 1977;
Kolla et al., 1980; Chamley, 1980a, 1980b). Lithologic,
mineralogic, micromorphologic, and geochemical observations suggest that northern Philippine Sea smectites
chiefly derive from the subaerial erosion and transport
of volcanic material. The submarine influence of volcanism on the clay formation is obvious only within the
basalts themselves, at the precise contact between the
basalts and the sediments (Chamley and Bonnot-Courtois, 1981), or in local ash layers marked by a larger exchange activity (Chamley, 1980a). Other diagenetic processes may affect the smectites, especially the growth of
lathed structures at the periphery of fleecy particles
(Plate 1). But these modifications are not necessarily related to a volcanic influence and can develop without
significant modifications of the clay mineral composition (Holtzapffel and Chamley, 1983).
638
Diversity of clay minerals, but homogeneity of the
sedimentary column (Figs. 2 and 5), characterize the middle Miocene to Pleistocene sedimentation at Site 584,
off the Kitakami massif of northern Honshu Island, on
the inner part of the Japan Trench. These attributes are
similar to those encountered for the same periods at
Sites 438 and 440 of Leg 57 (Fig. 5).
The clay mineral assemblages, marked by abundant
smectite, fairly abundant illite, chlorite and irregular
mixed-layers, and little kaolinite, are typical of the diversified detrital contribution issued from and deposited
around the Japanese islands (Sudo and Shimoda, 1978;
Kobayashi et al., 1964; Aoki and Oinuma, 1974; 1980;
Chamley, 1980b; Kolla et al., 1980). The absence of significant change in the average clay composition from
Site 438 to Sites 584 and 540 is due to similar distances
of the three sites from the Japanese coast (Fig. 1), precluding differential settling processes. The terrigenous
supply arrives at these sites primarily by marine transport, as suggested by the topography of the submarine
Japanese margin, by the high sedimentation rates (20 to
200 m/Ma; Leg 87 Scientific Party, 1983), and by the
presence of water-transported quartz (Le Mut, 1980). Eolian transport probably also occurs, as indicated by the
presence of common silt and clay-sized quartz (Fig. 5).
But this component is much less important volumetrically, than in eastern and northern parts of the northwest Pacific where wind-blown sediments are widely dispersed and easily recognizable in bottom samples (Leinen
and Heath, 1981; Rea and Janecek, 1982; Lenötre et al.,
in press).
The minor variations recorded in the clay mineralogic
composition since the late middle Miocene are chiefly
due to the effect of climatic change on the weathering of
land masses and to fluctuations in the position of the
main water masses carrying the suspended matter (Mann
and Müller, 1980). Parallel augmentations of illite and
chlorite, often associated with mixed layers and quartz,
probably correspond to cooler stages on land, whereas
increases of the smectite and kaolinite abundance indicate warmer stages. To deduce detailed climatic scale from
clay mineralogic stratigraphy would require denser samples. This lack of a sedimentologic data base thus prevents a clay mineral stratigraphy of Sites 584 and 438, a
stratigraphy that at present, mostly relies on diatom assemblages (Fig. 5). The influence of volcanism on clay
mineralogy appears to be restricted to the ash layers
(Cadet and Fujioka, 1980) and to not affect significantly the common muds and mudstones actively deposited since the middle Miocene.
CENOZOIC HISTORY OF THE NORTH JAPAN
MARGIN FROM CLAY STRATIGRAPHY
The clay assemblages off northern Japan change in
the middle Miocene. Smectite contents are higher in
Cores 584-94 to 584-97 (Fig. 5), but the main changes
occurred before the middle Miocene and were also recorded at Site 439 of Leg 57 (Scientific Party, 1980). We
LATE CENOZOIC PALEOENVIRONMENTS BY CLAY MINERALOGY
refer below to the clay stratigraphy established at Sites
438 and 439 (Chamley and Cadet, 1981) and consider
the evolution of the paleoenvironment in chronological
order (Figs. 7 and 8).
The Upper Cretaceous silicified and folded claystones
and siltstones occurring at the bottom of Site 439 contain abundant chlorite and illite (each mineral forming
40-50% of the clay fraction), with few mixed-layers (illite-smectite and illite-vermiculite), and common quartz,
feldspars, and amphiboles. Such a fairly simple assemblage does not correspond to the petrographic diversity
found on the Japanese islands and indicates the active
erosion of strongly sloped crystalline or metamorphic
rocks. The fine-grained rocks could have derived from a
continental landmass located east of Japan, at a fairly
long distance from the drilling area as indicated by the
fine grain-size of the sediments. This landmass could
have been a precursor of the Cenozoic "Oyashio landmass" (Scientific Party, 1980).
After a long hiatus during the major part of the Paleogene, the upper Oligocene deposits at Leg 57 sites
show three short mineralogic stages, marked by important variations (Fig. 7).
First, the lowermost clays and siltstones (1.5 m thick)
include the same upper Cretaceous minerals and also
noticeable amounts of well-crystallized kaolinite (20%
of the clay fraction) and abundant quartz. This compo-
Lithology
sition indicates the existence of a strongly eroded landmass different from Japan and contributing materials
that issued from both continental rocks and evolved soil
profiles. The landmass was probably the "Oyashio landmass" (Scientific Party, 1980; Von Huene et al., 1982),
located close enough to the drilling area to allow its identification from exoscopic and endoscopic studies of siltysandy quartz (Le Mut, 1980).
Second, the matrix of intermediate dacitic conglomerates (about 40 m thick) consists of highly crystallized
smectite (100% of the clay fraction) and abundant feldspars. The clay mineralogy indicates the rapid alteration
of volcanic materials, produced at about 23 Ma (Scientific Party, 1980). The large size of volcanogenic pebbles, together with the mineralogic pureness of their alteration products, point to the proximity of the effusive
activity, which was perhaps contemporaneous with the
subsidence of the Oyashio landmass.
Third, the uppermost Oligocene sediments consist of
fossiliferous sandstones and siltstones (about 100 m
thick). The clay mineralogy suddenly presents a diversified assemblage, including dominant smectite (40-80%
of the clay fraction), fairly abundant illite (10-30%) and
chlorite (5-25%), some irregular mixed-layers (traces up
to 5%) and kaolinite (traces up to 5%), abundant quartz,
common feldspars, and rare amphiboles. This assemblage resembles the clay suite derived from Japan dur-
Age
0
0-
Associated
minerals
Abundant
1007 Rare
Clay minerals
i
50
\
i
i
i
i
i
i
i
Pleistocene
I. Pliocene
5-
Sm
• . •
e. Pliocene
i
i
i
i
to
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^ > , CO
-5
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Miocene
CO J S
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middle
Miocene
CO
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15-
^
/
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O+
Sm
if
-
20- —
i
Δ O +
Δ
Φ
Δ O
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Turbiditic ^ —
clay/siltstone
Massive
2 5 - silt-stone/sandstone
N
Dacite
—
-— '
conglomerate clay
Of) _ ~Claystone/siltstone-
I
Hi
early
Miocene
Sm
+
θ
θ
Δ
Δ
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O +
o +
o +
•:•: •.-. ; -
Δ
:
late
Oligocene
O
1Δ
Sm
65Late
Cretaceous
i
i
i
i
i
i
i
i
i
+
o +
+
?
Hia tus
Claystone
Siltstone
Δ O
o
1 Δ OV
+
A
/
Feldspars x
Ar
nphiboles / Quartz
: Δ
O+
; Δ
O +
: Δ
O +
Figure 7. Major Cenozoic mineralogic assemblages of the Japan Trench inner slope (from Chamley and Cadet, 1981). For key, see Figure 2. Sm = smectite.
639
H. CHAMLEY, J.-P. CADET, J. CHARVET
West
Variable
temperate-humid
climate
,
sl
e
°P
East
Chlorite, Uite
mixed-layers,'
S R Λ
Pleistocene
Pliocene
Miocene
Deep-sea terrace
Inner trench slope
Trench
earliest
Miocene
Drilling at Site 584 ended in upper middle Miocene
diatomaceous mudstones, deposited after the major physiographic and paleoenvironmental modifications of the
eastern Japan margin. These sediments chiefly express
the water masses and climate changes, as do the sediments continuously and rapidly deposited above them
during the entire late Cenozoic. Clay diagenesis, locally
marked by lathed facies at the periphery of smectite sheets
(PI. 1, Fig. 6), remains of moderate importance, as it is
in all Cenozoic sediments rapidly deposited on the
northwestern Pacific margins.
ACKNOWLEDGMENTS
^ ' ^ m e d clay assemblage
4 3
£ T9 Ulite, ch»om«
J|
late
Oligocene
Strong erosion
Low pedogenesis
SW?
Illite, chlorite
NE ?
Crystalline
rocks /
North Japan
Honshu
Kitakami Massif
latest
Cretaceous
Northwest Pacific
Figure 8. Paleoenvironmental interpretation of Cenozoic clay stratigraphy off northern Japan (after Chamley and Cadet, 1981).
ing the late Cenozoic, but probably with a higher contribution of smectite altered from volcanic rocks or from
soils developed under a hot climate having strong seasonal contrasts in humidity (Chamley, 1980a; 1980b).
This mineralogic stage suggests that the first periods of
collapse and subsidence of the Oyashio landmass took
place as early as during the latest Oligocene and led to a
submarine morphology not very different from the Neogene, allowing the transport of Japan-derived minerals
to the drilling area of Legs 57 and 87B.
The lower to middle Miocene alternating clays, silts,
and sandy turbidites (about 200 m thick) do not show
clay mineralogic changes to be as important as the Paleogene deposits and primarily reflect the detrital supply
issued from Japan. Smectite abundance moderately increases at the transition from middle to upper Miocene
(Cores 438B-9 to 438B-6 and Cores 438A-85 to 438A-72),
possibly because of climatic changes (more arid, hot climate?). This period corresponds to the definite subsidence of the Oyashio landmass, according to geophysical, micropaleontologic and exoscopic evidence (Scientific Party, 1980; Le Mut, 1980). On the basis of clay
mineralogic data, this period must correspond to the end
of this major geodynamic event (Fig. 8).
640
We are grateful to the Leg 87 shipboard party for fruitful sampling
and stimulating discussions, to M. Bocquet, J. Carpentier, F. Dujardin, and P. Recourt for efficient technical assistance, and to W. Coulbourn and E. Whalen for very useful manuscript corrections. Financial support for the study was provided by Action Thematique Programmée Géologie et Géophysique des Oceans 1983 of the Centre National de la Recherche Scientifique, France.
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Date of Initial Receipt: 18 January 1984
Date of Acceptance: 14 June 1984
641
H. CHAMLEY, J.-P. CADET, J. CHARVET
1 µm
1 µm
1 µm
1 µm
1 µm
Plate 1. 1, 2, Nankai Trough; 582B-52-1, 53 cm; lower Pleistocene illite-rich and chlorite-rich assemblage; mostly well-shaped particles. 3, 4.
Nankai Trough; 582B-60-2, 70 cm; lower Pleistocene smectite-rich assemblage; mainly fleecy and lathed facies. 5. Japan Trench; 584-60-2,
18 cm; upper Miocene; illite, chlorite, and smectite in similar amounts; few kaolinite hexagons, debris of partly dissolved biogenic silica. 6.
Japan Trench; 584-96-2, 143 cm; middle Miocene; dominance of fleecy and lathed smectites, siliceous biogenic debris.
642